Top 10 Java Interview Questions You Must Know in 2025

🎯 Hook: Java might be over two decades old, but it's far from obsolete. With enterprise systems, Spring Boot APIs, and even Android still depending on it, Java continues to dominate backend job interviews — especially in Europe and North America.

In this article, we unpack 10 of the most common and high-impact Java interview questions for 2025. Whether you're a junior dev or prepping for a senior backend role, these are the questions you must be ready to answer — with confidence.

---

🔎 1. What’s the difference between var, final, and record in Java 17+?

Java has evolved. If you're still coding like it's 2012, it’s time to catch up.

Key points:

• var: Introduced in Java 10, allows local variable type inference.
• final: Makes variables or methods unchangeable.
• record: A concise way to create immutable data classes (Java 14+).

Why it matters: Interviewers want to see that you're using modern Java idioms — especially in clean, readable code.

---

🧠 2. How does the JVM manage memory? Can you explain garbage collection?

“Know thy JVM” — every serious Java dev’s mantra.

Cover:

• Stack vs Heap
• GC phases: Young, Old, Metaspace
• Modern GC types: G1GC, ZGC

🎯 Bonus tip: Mention memory tuning (-Xmx, -XX:+UseG1GC) if you're going for mid-to-senior roles.

---

🧱 3. HashMap vs ConcurrentHashMap — when and why?

You will be asked about this.

• HashMap is not thread-safe.
• ConcurrentHashMap uses lock striping for concurrency.
• In Java 8+, performance improvements via synchronized segments and compute functions.

🧠 Pro tip: Mention how ConcurrentHashMap avoids NullPointerException by disallowing null keys/values.

---

🌐 4. How do you build a scalable REST API using Spring Boot?

Expected topics:

• Layered architecture (Controller, Service, Repository)
• Async processing with @Async
• Exception handling with @ControllerAdvice
• Rate limiting, Swagger docs, and pagination

📦 Add points for integrating with Docker, Postgres, and caching with Redis.

---

🔁 5. What's the difference between == and .equals() in Java?

The classic trap: don’t confuse object identity with equality.

• == checks reference equality
• .equals() compares values (if overridden properly)
• Always override hashCode() if you override equals()

💡 Use a simple example with String or custom objects to make your answer memorable.

---

🔄 6. What’s a functional interface? Give real-world use cases.

Definition: An interface with exactly one abstract method, suitable for lambdas.

📌 Examples:

• Runnable
• Predicate<T>
• Function<T, R>

🧠 Interview bonus: Create your own custom @FunctionalInterface on the spot.

---

⚙️ 7. How does Java handle multithreading today?

• Use ExecutorService, ForkJoinPool, CompletableFuture
• Know Project Loom (virtual threads) – part of Java 21 LTS
• Mention thread pools, deadlock, and best practices for concurrency

🔥 Hiring managers love devs who can write safe, non-blocking code.

---

🤔 8. Optional, null, or try-catch — which to use when?

• Optional: Elegant null handling (Java 8+), especially for method returns
• try-catch: For exceptional behavior, not control flow
• null: Still exists, but handle with caution

✅ Interviewers care about your thought process — not just what you choose, but why.

---

🧱 9. What is the SOLID principle? Give an example in Java.

Break it down:

• Single Responsibility
• Open/Closed
• Liskov Substitution
• Interface Segregation
• Dependency Inversion

📦 Give a simple use-case like splitting service logic or using interfaces for abstraction.

---

📊 10. How would you process millions of records without crashing the JVM?

🛠️ Tools and techniques:

• Java Streams with .parallel()
• Paging with Spring Data
• Batch processing with Spring Batch
• Memory profiling with VisualVM or JFR

💡 Bonus: Talk about choosing streaming APIs over loading lists into memory.

---

🚀 Bonus Tips to Crack Your Next Java Interview

• Review recent JEPs (Java Enhancement Proposals)
• Practice on LeetCode, but focus also on system design
• Be ready to discuss real-world problems: legacy refactors, cloud migration, and microservices

---

🙌 Final Thoughts

Java isn’t going anywhere — and interviews in 2025 are tougher, more real-world, and more architectural than ever. By mastering these questions, you’re not just prepping answers — you’re learning to think like a Java backend engineer.

📬 Want more Java deep dives, Spring Boot patterns, and interview prep kits?

Subscribe to the newsletter →

---

FAQs

Which Java version should I prepare for interviews in 2025?

Java 17 (LTS) and Java 21 (latest LTS) are your best bets.

Do I need to know Spring Boot to pass Java interviews?

For backend roles — absolutely. It's the default framework in 90% of modern Java projects.

Should I learn Java or Kotlin for Android interviews?

Kotlin is preferred for new Android dev, but Java knowledge is still required in most legacy projects.

How technical are Java interviews today?

Expect a mix of theory, live coding, system design, and even DevOps if you're aiming mid/senior level.

---

🧠 Remember: It’s not just about memorizing answers. Show how you think, how you debug, and how you architect real-world solutions.

Ready to code? 💻 Let’s go.

---

📚 Further Reading & Resources

Here are some helpful links and official docs to dive deeper into Java interview prep in 2025:

• Java Interview Questions
• 56 Java Interview Questions And Answers For All Levels
• Java Interview Questions and Answers for Beginners